1. Field of the Invention
The present invention relates to improved insulated high voltage cables. More particularly, this invention relates to an improvement in cable design which will reduce dielectric losses within the total cable construction.
2. Description of the Prior Art
Ordinary insulated cables that are designed to carry high power loads, e.g., 2000 to 130,000 volts, are subject to a serious drawback in that imperfections and especially imperfections which are voids are apt to occur between the conductor and the insulation and between the insulation and outer shield. The electrical degradation which occurs at these imperfections is manifested by ionization and possibly other electrical phenomenon and results in a rapid breakdown of the insulation at these imperfection points. The breakdown is manifested by severe dielectric losses or complete failure of the insulation through the so-called "treeing" phenomenon.
U.S. Pat. No. 3,287,489 to Hvizd, Jr., the disclosure of which is incorporated herein by reference, describes a means for combatting the disadvantages associated with conventional insulated cables by insulating the conductor of a high voltage cable with a laminar insulating material of specific construction. The laminar insulation includes a thick layer of insulation material of low specific inductive capacity (hereinafter SIC), e.g., within the range of about 2 to about 4.5, and a thin layer of insulating material of high SIC, e.g., within the range of about 10 to about 25.
Thus, according to the teachings of the Hvizd, Jr. '489 patent, there is provided an insulated high voltage cable comprising a central core of metal of high conductivity and an outer metallic shield. Laminar insulation is located between the core and the shield. Such laminar insulation includes a thick layer of insulating material of low specific inductive capacity and a thin layer of insulating material of high specific inductive capacity covering at least one face of the thick layer of low specific inductive capacity insulating material.
The explanation of the property of "high" SIC, and an SIC value increasing with increasing temperature, lies in a basic characteristic of certain polymers known as "dipole moment." Certain polymers contain polar molecules which exhibit a dipole moment. This polymer structure characteristic is well known and is due to a particular type of atom or group of atoms, such as a halogen, having a charge, and being so arranged spatially to allow movement in an alternating current field. A measure of the effect of an element or group's dipole moment in an AC field has been referred to loosely as "molecular friction," an indication of which is SIC and power factor. Thus, hydrocarbon polymers without polar molecules have a "low" SIC, e.g., less than 4 at room temperature, and are temperature stable. Those polymers containing polar molecules have a high room temperature SIC (4 to 12) and are not stable with increasing temperature, i.e., some have a positive temperature coefficient and some a negative temperature coefficient.
The Hvizd, Jr. '489 patent did not assign values to the power factor or tan .delta. of the insulation layers as this was considered a property that was related to the SIC in formulating the insulation composition. In fact, the tan .delta./SIC ratio of the insulation layers disclosed in that patent over the operating temperature range fell between 0.006 and 0.022. This can result in dielectric losses in the high SIC layers approaching and exceeding the loss in the primary insulation, depending on the primary insulation used and its relative thickness compared to the normally thinner high SIC layers. Dielectric losses increase the wattage losses in the cable and thereby increase the cost of transmitting electrical power. Although the dielectric losses for a short length of cable may not be highly significant, in applications which require the cable to have lengths in excess of one mile, these losses are cumulative over the cable length and can significantly affect the feasibility of a particular cable for such applications from a cost standpoint.